1. Field of the Invention
The present invention relates to a method of manufacturing a group III-V compound semiconductor solar battery such as a gallium arsenide (GaAs) solar battery.
2. Description of the Prior Art
In recent years, studies have been made to increase the photoelectric conversion efficiency of silicon (Si) and group III-V compound semiconductor solar batteries. Through such studies, it has been found that a GaAs solar battery can attain higher improvement in conversion efficiency than a Si solar battery and that such an effect is further increased when group III-V ternary compound is employed with GaAs to form the solar battery. In the latter case, generally an n-type GaAs layer of excellent crystal quality is epitaxially grown on an n-type GaAs crystal substrate which is prepared by slicing n-type GaAs crystals obtained by the horizontal Bridgeman method (HB) or the liquid encapsulated Czochralski method (LEC), and a p-type GaAs region is formed in the surface part of the n-type GaAs layer to form p-n junction, and then a p-type Al.sub.x Ga.sub.1-x As layer is further epitaxially grown on the same. Such a semiconductor device is manufactured by employing a liquid-phase epitaxial (LPE) growth technique.
For example, an n-type GaAs saturated solution formed with Ga as a solvent, GaAs as a solute and tellurium (Te) as an n-type impurity and a p-type Al.sub.x Ga.sub.1-x As saturated solution formed with Ga as a solvent, GaAs and Al as solutes and Mg as a p-type impurity are placed in the same crystal growth boat. Then, an n-type GaAs crystal substrate prepared by slicing n-type GaAs crystals obtained by the HB method or the LEC method is dipped in the n-type GaAs saturated solution and then in the p-type Al.sub.x Ga.sub.1-x As saturated solution at respective prescribed temperatures which are lowered to respective prescribed temperatures for crystal growth after respective prescribed retaining times. Such crystal growth steps are continuously performed in the same crystal growth boat.
Namely, after completion of n-type GaAs epitaxial growth, the n-type GaAs crystal substrate dipped in the n-type GaAs saturated solution of Ga and GaAs containing the n-type impurity is separated from the n-type GaAs saturated solution and then continuously dipped in the p-type Al.sub.x Ga.sub.1-x As saturated solution of Ga, GaAs and Al containing the p-type impurity and retained for the prescribed time so that the surface part of the n-type GaAs epitaxial layer is inverted to a p-type to form a p-n junction portion serving as an operation layer of a GaAs solar battery. Then, the p-type Al.sub.x Ga.sub.1-x As saturated solution is lowered to the prescribed temperature to epitaxially grow a p-type type Al.sub.x Ga.sub.1-x As layer.
In the aforementioned manufacturing method, however, the characteristics of the GaAs solar battery are largely fluctuated by the number of times of crystallization. Namely, since the n-type GaAs saturated solution adhered to the substrate is mixed into the p-type Al.sub.x Ga.sub.1-x As saturated solution when the substrate completed with n-type GaAs epitaxial growth is dipped in the p-type Al.sub.x Ga.sub.1-x As saturated solution, the p-n junction characteristic cannot be stably controlled and the electric characteristic of the GaAs solar battery is deteriorated as the number of times of crystallization is increased.